A stereo audio recording provides the ability to place apparent sources of sound on the listener's left, right, or anywhere in between, but only the extreme left or right positions are “pure”. Sounds that appear to come from a location between the left and right speakers are actually reproduced in varying proportion by both speakers resulting in a “phantom image.” Since the introduction of stereo recording, audio enthusiasts have sought to enhance the listening experience by adding more positional channels, usually a front-center and one or more positions behind the listener. While innumerable attempts have been made toward this end, none have successfully isolated apparent sound sources. Movie theaters and home theaters now often employ four or five channel audio. When provided with multi-channel inputs appropriate to their design, these multi-channel audio systems can recreate a soundstage with great accuracy. However, multi-channel systems are not fully utilized when provided with only conventional stereo input. It is desirable to provide a method of processing the left and right channels of a stereo recording to be able to create electrical signals for three or more channels and more effectively utilize the capabilities of multi-channel audio systems.
In the late 1960's and early 1970's visual display devices (“color organs”) were popular. Color organs are commonly driven from the same signal source as the loudspeakers. The devices were connected to the left and right speakers and displayed colored light patterns corresponding to the audio signal being fed to the speakers. Such a connection often leads to incongruous audio-visual presentations. In the simplest and most glaring case, the two loudspeakers in a stereo system are fed by in-phase signals of equal amplitude. The resulting sound appears to be originating from a point in space midway between the two loudspeakers, but the color organs produce a display on the extreme left and extreme right. There is no display in the center where the sound is, and there is no sound on the left and right where the displays are.
Ideally, the visual display should occur where the sound appears to originate. In the foregoing example, the display would appear at a point midway between the speakers and no display would appear on the left or right. More generally, as a sound appears to move from one side of the stereo soundstage to the other (left-to-right or right-to-left), the visual display should move accordingly.
A method is needed to generate a display that coincides with the apparent source of sound. Several methods such as rapidly moving a (possibly multi-colored) laser beam or generating an image for display on a cathode ray tube, for example, are possible. The method of the current invention employs a plurality of individual display devices arrayed across the area encompassed by the left and right loudspeakers. Each of the display devices is provided with its own input. The current invention can generate these inputs from the signals comprising the left and right stereo channels.
In addition, the current invention is more than simply a device to drive a visual display; it is also a multi-channel from two-channel audio playback device, as the inputs for an array of color organs can be used to generate an array of audio channels to be played over a group of loudspeakers.
It is therefore an object of the invention to apply a process to a pair of stereo audio-frequency input signals to produce at least three and preferably about eight audio-frequency output signals representing the left, left-center, center, right-center, and right positions in the input stereo soundstage (in front of a listener) and three positional channels behind the listener (right-back, center-back, and left-back).
It is a further object of the invention that each of the output channels of the current invention be made to be discrete. That is, a signal appearing in one output channel appears in at most one adjacent output channel. Having a signal in two adjacent channels is necessary to allow the formation of phantom images between the said two adjacent output channels. All other channels have no output of that signal. If the position of the phantom image in the input stereo soundfield is coincident with one of the output channels, then only that channel produces an output signal. All other channels produce no output of that signal.
It is yet another object of the invention that the rear channels be utilized in ambience recovery, but unlike prior-art systems with similar purpose, the rear channels are correlated and directional, and are thus fully capable of forming phantom images. In this fashion, it is possible to produce a stereo (two channel) recording wherein a sound source appears to be coming from a particular position behind the listener as the sound from the rear channels is not diffuse.
It is still another object of the invention that in addition to audio-frequency output signals, the current invention provides a plurality of output signals dedicated to the generation of a visual display; a “Color Organ.” Each of the front output channels may have a corresponding color organ output. Optionally, an odd number (1, 3, 5, 7, . . . ) of color organ outputs may be generated between adjacent audio-output channels. By design, the process does not produce color organ outputs corresponding to the rear channels. This is not a limitation of the method. If rear color organ outputs are desired in a particular embodiment of this invention, they may be generated in the same manner as the front color organ outputs. Conventional color organs may be fed from the audio-frequency outputs, or a novel color organ developed to employ these outputs
Fulfilling these objectives of the invention is not trivial. Stereo recordings are routinely produced through a process known as “pan-potting” whereby the intensity of the left and right signals is varied to affect positioning of the sound anywhere between the left and right loudspeakers. By applying the mathematical inverse of the pan-potting equations, the original signal at a given position may be recovered. Unfortunately, the inverse operation is only possible at a single frequency.
The present invention circumvents this difficulty by subjecting the input signals to a Fast Fourier Transform (FFT) which, in essence, converts them to a plurality of narrow frequency bands to which the inverse pan-potting equations may be applied. The result is not exact, but if the number of frequency bands is high, their width is sufficiently narrow to produce acceptable results. This processing is computationally intensive and may be implemented by software to convert digitized stereo data streams or files into data for each of three or more audio channels. Preferably, the computational instructions can be imbedded in processors and digitized stereo data converted to three or more digitized audio output and/or color organ output channels on the fly.
The number of locations in the stereo soundfield where the inverse pan-potting equations are to be applied and the left-right position of these locations can be chosen arbitrarily. In the currently preferred embodiment, this invention employs five equally spaced front locations. Input signals that are more than 90 degrees out-of-phase are deemed to belong in the rear and are assigned to the back channels. Signals on the extreme left or right are not reassigned, thus generating three (five front minus left and right) channels in the rear. This is considered adequate for most implementations, but the process is not limited to these values.
After the inverse pan-potting equations have been applied to each frequency band in the left and right FFTS, the result is the Fourier Transforms of the sound sources, if any, present at each of the chosen locations. The color organ outputs are derived directly from these Fourier Transforms because signal intensity at different frequencies is often an important parameter in color organs. The audio-frequency output is generated by taking the Inverse Fourier Transform (IFT) of each output channel.
Additional processing steps such as echo removal, signal enhancement, or gain riding may be performed prior to taking the IFT, but are not essential to the proper functioning of the invention. Further processing may also be done after the IFT is taken. The present invention provides an electronic channel separator device intended to enhance stereo audio playback by isolating apparent sound sources across the stereo sound stage and feeding these isolated sound sources to separate channels of amplification. Most of the room reverberation (echo, ambience) present in a recording appears in channels behind the listener. In addition, outputs are provided to drive a visual display (“color organ”) to be placed in front of the listener. The visual display appears at the apparent location of the sound, and is not sensitive to room reverberation (It only responds to the primary elements in the recording). The channel separation device is connected between a conventional stereo audio signal source and one or more conventional audio amplifiers. If a graphic equalizer is employed in the playback system, the channel separation device should be connected between the graphic equalizer and the amplifier. No special hardware or adapter need be required, as suitable connections may be made through standard audio cables. The audio source may be any device that emits a stereo signal, such as a CD player, DVD player, mp3 player, tape player, radio tuner, computer disk drive or phonograph. The amplifier (or amplifiers) must have at least as many channels of amplification as the channel separation device produces. The channel separation device may be designed to produce any number of channels, but fewer than three channels accomplishes no purpose and more than eight channels is of little utility owing to the nature of recorded music Conventional stereo recordings rarely use more than eight stage positions. Owing to the present predominance of the Dolby AC-3 (5.1) home theater system, the channel separation device may be designed to supply audio outputs compatible with the Dolby 5.1 system, thereby facilitating continued, but enhanced, use of existing audio equipment. While the channel separation device is intended for use in private homes, it may find wide utility in public venues such as concert halls, arenas, stadiums, amphitheaters, planetariums, and museums. The ability to locate the apparent source of sound in a stereo recording is degraded when the listening space is large, as it is in public venues. The degradation increases as the size of the venue grows larger. Isolating apparent sound sources and routing them to actual sound sources (loudspeakers) greatly improves the reproduction quality. Furthermore, planetariums often include a light show based upon recorded music as part of their presentation and may utilize both audio channels and color organ channels for this purpose Movie theaters often play recorded music prior to the feature film and could benefit from the addition of a visual display component.